Selective Oxidation of Cyanotoxins (Microcystins) by Peroxyacetyl Radical: Kinetic Studies and Mechanistic Considerations

Microcystins (MCs) are one group of the most toxic cyanobacterial metabolites in surface waters, yet their selective and efficient removal under environmentally relevant conditions remains a major challenge. Here, we identify the peroxyacetyl radical (PAR) as a previously unrecognized oxidant for MC degradation. We first quantified the bimolecular reaction rate constants of PAR with 10 MC congeners, which fell within a narrow range of (3.94–6.69) × 107 M–1 s–1. Our results revealed that PAR selectively attacks the conjugated diene in the Adda side chain, yielding dihydroxylation products while leaving the aromatic ring and Mdha largely unreacted. High-resolution MS/MS confirmed the formation of three dominant dihydroxylated products, accounting for ∼80% of total yield. The environmental relevance of PAR was further demonstrated in solar/peracetic acid (PAA) systems, where MCs were degraded mainly via two parallel pathways: HO• oxidation and PAR oxidation. Quantitative radical budget analysis revealed that PAR contributed 1.5–3.5 times more to MC degradation than HO• across varied PAA/H2O2 ratios and DOM levels. Overall, this study provides the first mechanistic and kinetic evidence that PAR is a selective oxidant capable of efficiently degrading MCs under sunlight and realistic water matrices, highlighting its potential for next-generation cyanotoxin treatment.